The upcoming Ubuntu 21.04 release will not be shipping with GNOME 40 or GTK4. This is due to the redesign of GNOME Shell in GNOME 40 and its potential impact on GNOME extensions some of which are shipped with Ubuntu by default.
GNOME 40 comes with several major Shell user experience. These design changes will directly conflict with Ubuntu. Because of this time will be needed to reintroduce the Ubuntu experience in GNOME.
On the other hand, GTK4 comes with several new capabilities for app developers and designers that impact third-party themes like Yaru. Again this will need to accommodate all the changes.
GNOME 3.38 point release and GTK3 apps with the Yaru GTK theme will continue to be the flavor of this new Ubuntu release.
Read more about the changes coming in GNOME 40 here.